[Diy_efi] A 2N2222A can handle up to 50mA

[Perry Harrington]

Bevan,

"only 8", who wants to drive 8 of these off a single port? If it
concerns you, drive them off another transistor with a pullup.

This discussion has gotten way away from the original application: driving
a relay.

The chip driving the transistor can put out 20-25ma of drive current (LM339),
so there is no issue with a standard 2N2222, much less the darlington.

The darlington suggestion was due to a single device driven off an MCU.

Of course all applications will vary depending on the MCU or other driver.

--Perry

> > An even simpler and cheaper solution is one of the following:
> >
> > MPSA27 (TO-92)
> > MPSA28 (TO-92)
> > NTE 46 (TO-92)
> >
> > Those are 500ma Darlingtons at about 14 cents a piece in bulk.  They have
> > a gain of 10,000.  Base current is then unimportant.
> 
> Base current doesn't become unimportant with this gain, that means that an
> Ic of 500mA will see a base current of 50uA.  Now this may seem pretty low,
> but it means that you can only drive 8 of these from a standard digital
> source (assuming source current max = 400uA typical).  A FET is my preferred
> way of interfacing any digital signals... it has an incredibly low leakage,
> and a very fast turn on time (depending only on gate capacitence and drive
> resistance).
> 
> > Digikey doesn't have any of the above in stock, but they do have the
> MPSA63,
> > it's rated for max continuous collector current of 1.2A and has a gain
> > between 5,000 and 10,000.  It's also 26 cents for 1, 19.8 for 25, and
> 13.73
> > at 100.
> 
> If it's a DIY thing, then price doesn't play as big a role as it would in
> OEM decisions.  The better design decisions can be made without compromise.
> If you're dealing with digital devices (which use voltage levels for
> signalling) then it makes sense to use a voltage sensing device as the
> switch.

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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